THRIFT-4591: optimize lua client to use one write per framed message (#1566)

diff --git a/lib/lua/TFramedTransport.lua b/lib/lua/TFramedTransport.lua
index 437b701..768e2d9 100644
--- a/lib/lua/TFramedTransport.lua
+++ b/lib/lua/TFramedTransport.lua
@@ -100,7 +100,7 @@
   local tmp = self.wBuf
   self.wBuf = ''
   local frame_len_buf = libluabpack.bpack("i", string.len(tmp))
-  self.trans:write(frame_len_buf)
+  tmp = frame_len_buf .. tmp
   self.trans:write(tmp)
   self.trans:flush()
 end